Our recent discovery of MTNR1B, encoding for the high-affinity melatonin receptor MT2, as a novel genetic risk factor for type 2 diabetes (T2D) and glycemic traits in genome wide association studies (GWAS) has sparked great interest in the role of melatonin in glycemic control. However, the exact mechanism is not yet understood. Two major limitations in all previous GWAS are that: 1) the clinical assessment of glycemic traits is based on daytime assessments, when circulating concentrations of the ligand of the MT2 receptor, i.e., melatonin, are near-undetectable; and 2) that the impact of the MTNR1B risk variant has not been assessed in vulerable populations with elevated endogenous melatonin concentrations concurrent with food intake, such as shift workers and natural late night eaters. Our preliminary data suggests that exogenous melatonin administered during the daytime acutely decreases glucose tolerance in healthy participants, with an exaggerated response in risk carriers, and the increase in the odds ratio for T2D with shift work is 5-fold higher in MTNR1B risk allele carriers as compared to the non-carriers. Our long term goal is to use mechanistic understanding of the role of melatonin and MTNR1B in glucose metabolism to design interventional strategies for the prevention and treatment of T2D, particularly in vulnerable populations, such as night shift workers and late night eaters. Our objectives are to: 1) determine the effect of concurrent food intake and melatonin on glucose tolerance; and 2) assess the role of MTNR1B SNP*food timing interaction in the deleterious effect of melatonin on glucose tolerance. In this proposal, we address two specific aims: 1) test the hypothesis that the concurrence of meal timing with elevated endogenous melatonin concentrations associates with adverse glycemic changes in shift workers versus non-shift workers, including elevated T2D risk, decreased glucose tolerance and decreased disposition index, and that these effects will be magnified in risk carriers; and 2) test the hypothesis that the concurrence of meal timing with elevated endogenous melatonin concentrations associates with decreased glucose tolerance and decreased disposition index in late night eaters versus early eaters, and that these effects will be magnified in risk carriers. These hypotheses will tested in a large scale cohort with information on T2D risk and shift work history (UKBiobank, n=287,635); and two new cohorts, shift workers versus non-shift workers (US; n=1,000) and late night eaters versus early eaters (Spain; n=1,000), with detailed information on meal timing, endogenous melatonin concentrations, as well as morning (low endogenous melatonin levels) and evening (high endogenous melatonin levels) OGTTs. Completion of this project will lead to important novel insights into the role of MTNR1B, melatonin and food timing on glucose regulation. This work is also expected to result in personalized recommendations to reduce risk of T2D in carriers of the MTNR1B risk allele, translating GWAS and physiologic research into a practical clinical application.